Development and applications of the interfacial tension between water and organic or biological surfaces.

The interfacial tension (gamma(SW)) between a condensed-phase material (S) and water (W) is one of the most important terms occurring (directly or indirectly) in the major surface thermodynamic combining rules, such as the different variants of the Dupré equation, as well as the Young and the Young-Dupré equations. Since the late 1950s, gamma(SL) (where L stands for liquid in general) could be correctly expressed, as long as one only took van der Waals attractions and electrical double layer repulsions into account, i.e., as long as both S and L were apolar. However for interfacial interactions taking place in water among apolar as well as polar solutes, particles or surfaces, gamma(SW) was not properly worked out until the late 1980s, due in particular to uncertainties about the treatment of the polar properties of liquid water and other condensed-phase materials. In this review the historical development of the understanding of these polar properties is outlined and the polar equation for gamma(SW), as well as the equations derived there from for the free energies of interaction between apolar or polar entities, immersed in water (deltaG(SWS)) are discussed. Also discussed is the role of the various terms of deltaG(SWS), in hydrophobic attraction (the "hydrophobic effect"), hydrophilic repulsion ("hydration forces") and in the quantitative expression of hydrophobicity and hydrophilicity. The DLVO theory of attractive and repulsive free energies between particles immersed in liquids, as a function of distance between suspended particles, was extended to allow its use in the expression of the polar interactions occurring in water. Finally, the free energy term, deltaG(SWS) and the related gamma(SW), have been directly linked to the aqueous solubility of organic and biological solutes, which allows the determination of interfacial tensions between such solutes and water from their solubilities.

Long-range and short-range mechanisms of hydrophobic attraction and hydrophilic repulsion in specific and aspecific interactions.

Among the three different non-covalent forces acting in aqueous media, i.e. Lifshitz-van der Waals (LW), Lewis acid-base (AB) and electrical double layer (EL) forces, the AB forces or electron-acceptor/electron-donor interactions are quantitatively by far the predominant ones. A subset of the AB forces acting in water causes the hydrophobic effect, which is the attraction caused by the hydrogen-bonding (AB) free energy of cohesion between the water molecules which surround all apolar as well as polar molecules and particles when they are immersed in water. As the polar energy of cohesion among water molecules is an innate property of water, the hydrophobic attraction (due to the hydrophobic effect) is unavoidably always present in aqueous media and has a value of DeltaG(hydrophobic) = -102 mJ/m(2), at 20 degrees C, being equal to the AB free energy of cohesion between the water molecules at that temperature. The strong underlying hydrophobic attraction due to this effect can, however, be surmounted by very hydrophilic molecules and particles that attract water molecules more strongly than the free energy of attraction of these molecules or particles for one another, plus the hydrogen-bonding free energy of cohesion between the water molecules, thus resulting in a net non-electrical double layer repulsion. Each of the three non-covalent forces, LW, AB or EL, any of which can be independently attractive or repulsive, decays, dependent on the circumstances, as a function of distance according to different rules. These rules, following an extended DLVO (XDLVO) approach, are given, as well as the measurement methods for the LW, AB and EL surface thermodynamic properties, determined at "contact". The implications of the resulting hydrophobic attractive and hydrophilic repulsive free energies, as a function of distance, are discussed with respect to specific and aspecific interactions in biological systems. The discussion furnishes a description of the manner by which shorter-range specific attractions can surmount the usually much stronger long-range aspecific repulsion, and ends with examples of in vitro and in vivo effects of hydrophilization of biopolymers, particles or surfaces by linkage with polyethylene oxide (PEO; also called polyethylene glycol, PEG).